This invention relates to a Geneva gear driving apparatus for an index table which can be used in a replaceable gang head machine tool.
Various Geneva gear driving apparatuses are known in the art. For example, a driving apparatus for an index table in a replaceable gang head machine tool is shown in U.S. Pat. No. 4,216,572. This apparatus is constructed and operates as shown diagrammatically in FIG. 1.
A Geneva pin c is connected to a driving shaft a through a crank b. The pin c is engaged in the Geneva gear grooves e in an index table d. The grooves have force bearing walls comprising straight portions and sector portions. The table d is rotated to a predetermined angular position (through an angular turning of 90.degree. in the illustrated example) upon each rotation of the driving shaft a. It is usual with this type of apparatus that the distance between the pin c and the axis of the driving shaft a is set to be equal to the distance L between the axis of the driving shaft a and the centerline of the straight portion of the groove e with the circumferential surface of pin c bearing against the force bearing walls of the groove e. When the driving shaft a and the crank b are rotated, the pin c powers the rotation of the index table d by bearing against the straight portions of the groove e. When the table d reaches the desired predetermined angular position, the pin c transfers from a straight portion of groove e to a sector portion and consequently, the driving of the table d by the pin c is discontinued
The above-described apparatus has a problem when a large mass article such as a gang head or the like is carried by the table d. When the pin c transfers from a straight portion of groove e to a sector portion, the table d continues to rotate because of the inertia of the large mass being carried. A positioning member f such as a positioning pin or the like has been provided in prior art apparatuses to prevent such excessive rotation. The pin f is brought into engagement with one of the grooves e in the table d when the table reaches the desired angular position. This has been known as a Geneva stop mechanism. However, the positioning member f can be damaged by the stress generated upon engagement with the groove because of the inertial force of the table and the large mass article carried thereon. The table d can also be undesirably stressed. Additionally, an inaccuracy in the position of stopping of the table can result.
An object of the present invention is to provide a Geneva gear driving apparatus for an index table which will smoothly and accurately drive the index table between desired, predetermined angular positions.
It is a further object to provide an apparatus which utilizes the inertia of a moving index table carrying a large mass article to complete the rotation of the table to desired, predetermined angular positions.
It is a yet further object to provide an apparatus which avoids overstressing a positioning pin which holds the index table at a desired, predetermined angular position.
It is a still further object to provide an apparatus which is adjustable for different-mass articles to be carried by an index table to desired, predetermined angular positions.
In accordance with the above objects and others which will become apparent to those of skill in the art upon reading the description give hereinbelow, the present invention is an apparatus for driving an index table comprising
a first shaft;
an index table rotatably mounted on the first shaft, the table having grooves therein forming a Geneva gear, the grooves having force bearing walls comprising straight portions and sector portions; and
means for driving the table in rotation including a drive motor, a second shaft driven by the motor having an axis, a crank mounted on the second shaft, and a Geneva pin mounted on the crank having an axis and a circumferential surface, the pin being engaged in the grooves in the table to effect stepwise rotation of the table to equispaced, predetermined angular positions; wherein
the second shaft and the grooves are spaced from each other such that the distance from the axis of the second shaft to the nearest portion of the circumferential surface of the pin is less than the distance from the axis of the shaft to the straight portions of the force bearing walls of the grooves and wherein the sector portions of the force bearing walls of the grooves are recessed such that the distance from the axis of the second shaft to the nearest portion the circumferential surface of the pin is equal to the distance from the axis of the second shaft to the sector portions of the force bearing walls of the grooves,
so that the circumferential surface of the pin transfers from bearing against the wall of a straight portion to bearing against the wall of a recessed sector portion of the groove prior to the index table finishing rotation to one of the predetermined angular positions, and the index table completes rotation to the predetermined angular position powered solely by its inertia.
Additionally, the apparatus can further comprise a vertically movable positioning pin positioned and aligned to engage one of the grooves by entering the straight portion thereof when the index table is at the predetermined angular position.
An adjusting means mounting the Geneva pin on the crank for adjusting the distance between the axis of the Geneva pin and the axis of the second shaft can be provided.
The adjusting means can comprise a bushing rotatably mounted in the crank having an eccentric opening therein in which the Geneva pin is mounted.